Bore sight for insertion in the breech chamber of guns to provide overlapping imagesof muzzle portions of the gun bores



June 7, 1949. 4 BROWN BOR IGHT FOR INSERTION IN THE BREECH CHAMBE GUNS TO PROVI OVERLAPPING GES OF MUZZLE PORTI OF THE GUN RES Filed May 23, 1944 iig-L 1 EN X 6 :l 6 2 :rf/6

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Patented June 7, 1949 UNITED STATES PATENT OFFICE` BORE SIGHT FOR INSERTION IN THE BREECH CHAMBER OF GUNS TO PRO- VIDE OVERLAPPING IMAGES OF MUZ- ZLE PORTIONS OF THE GUN BORES (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of yany royalty thereon.

The present invention relates to a bore sight for guns or cannon.

A bore sight is used to determine whether the line of sight of an external sighting device of a gun or cannon is parallel with the axis of the bore of a gun or cannon. A bore sight is also used in adjusting the line of sight of an external sighting device of a gun or cannon into parallelism with the axis of the bore of the gun or cannon. Thus, for example, Where a telescopic sight or a collimator sight or a reex sight is used as the external sighting means for cannon, a bore sight is used to determine whether the line of sight of the telescope, collimator sight or reex sight is adjusted parallel to the axis of the bore of the cannon.

Heretofore in determining whether this parallelism existed or in establishing it, two lengths of cord or thread were stretched, at right angles to each other, coincident with diameters across the bore at the muzzle end of the cannon or gun, and a disk which exactly fitted the chamber of the cannon or gun, and having a small hole therethrough at its center, was inserted within the chamber of the gun or cannon through its breech. The line or line of sight between the intersection of the two lengths of cord and the small hole through the disk substantially coincides with the axis of the bore of the cannon, With the breech end of the gun open, one sighted through the bore on a distant target, placing the center of the opening in the disk and the intersection of the cords in line with the target or a selected portion thereof. Then the external sighting device of the cannon or gun, such as a telescopic sight, or a collimator sight, was sighted on the same target or on the same portion or point of the target. This placed the line of sight of the external sighting device substantially parallel with the axis of the bore of the gun.

One of the main objections to the above mentioned procedure of bore sighting is that the cords must be stretched across the muzzle end of the bore. Where for example, a cannon is mounted on a combat tank, this placing of the cords across the bore of the cannon at the muzzle end is only an inconvenient procedure, requiring personnel manning the tank to leave their posts within the tank and to mount upon the exterior of the tank to placey the cords in position, but also the procedure is a hazardous one during combat or within the field of operations.

Considering the rough terrain over which combat tanks are required to travel, it will readily be appreciated that the gun sights are very much subject to being jarred out of adjustment, and the necessity of bore sighting may be frequent.

It will, therefore, be understood that it is of importance to be able, easily and effectively, to place the line of sight of the sighting means for cannon mounted on a combat tank linto parallelism with the axis of the bore of the cannon, or to determine whether such parallelism exists, without it becoming necessary for personnel manning the tank to leave their posts in the `interior of the tank. One of the objects of the present invention is to provide apparatus, or a bore sight, which enables this to be done. Another object of the invention is to provide apparatus of the character referred to which is easy or simple to use, and which in addition is of simple construction and rugged.

The here sight of the present invention includes means for sighting, from the breech end of a gun or cannon along the surface of the bore, along a line parallel with the axis of the bore, to obtain a view of at least a part of the periphery of the muzzle end of the bore; also another means, which is spaced circumferentially of the bore from the sighting means previously referred to, for also sighting, from the breech end of the cannon, along the surface of the bore, along another line parallel with the axis of the bore, so as to obtain another View of at least a part of the periphery of the muzzle end of the bore, and means for overlapping the twoviews so obtained.

When the above mentioned views are overlapped, the circular peripheries, or the portions of the circular peripheries, of the muzzle end of the bore of the cannon intersect, and their point or points of intersection establishes a line of sight, through the bore, parallel to the axis of the bore. The barrel of the gun or cannon is now moved in azimuth and in a vertical plane so as to place the said intersection in the overlapped views on a selected distant target or on some particular point thereon. The external sight of the cannon is now adjusted s0 that its line of sight is on the same target or on the saine selected point thereof. After this adjustment has been made, the line of sight of the external sight is parallel with the axis of the bore of the cannon or gun.

The drawings accompanying this description, and forming part thereof, illustrate one embodiment of the invention. In these drawings Wherein the same reference character indicates the same or similar parts,

Fig. 1 is a longitudinal section of a part of a cannon barrel along a plane parallel with but removedltrom the-.zaxiso' the bore of the cannon, and showing; partly in section and' in plan, a bore sight according to the invention placed in position in the chamber of the cannon for use in bore Figs. 5, 6 and 7 are typical -examplesfofftwo overlapped views referred to above, and. indicate elds of view as seen by a person using the sight.

Referring now in detail to the" drawings, the

numeral I indicates the barrel of a cannon, havingY the. bore 2i 'andi the chamber 3.

The bore sight comprisesv a. frame or disk'5 whichaccurately fits with la sliding nt against tl:`1e1.iei-'ipherye of the chamber. 3. The chamber 3 maytaper slightlyftowardsthe muzzle, so that thedisk 5.1mayfreach. a4 position in the chamber suchfthat further movement towards the muzzle is-not possible- @This is not; disadvantageous because;it.ispreferable= to insert the disk 5 only a; shortdi'stance inY the: chamber 3.

Over afchordiofz thefdisk'5 is secured a housing Iifcontaining prisms; 1f. and4 8.. Prisms l and 6 may be' of glass forl 'other light." transmitting material andf are of theesameiindex of refraction. Prism 'I isf cementedV tor prism. Il'V by a l transparent cement having the-.samer or about the. same index of refractionas'prismsu'l and'8..V Before prisms l' and 8 are; cemented; together, oneoftthe faces of them forming. theinterface 9 is provided. withv a halfsilveredmirronfthat isto.- say', a, mirror that refleets about 50%4 of'lightincident thereon and transmits 50% of the saidi-ncident light. Said mirrorsurfacainstead of being formed of silver, may be. formed ofz'anyother suitable substance providing a coating which .partially reflects light andfpartially transmitslight. There is, therefore, inthe-.interface 9a betweenv the lprisms 7 and B2 a mirroror. reflector which partially reflects and partially: transmits light.

The interface 9 andi the. face I6 of the prism 8 arevparallel andfea'chAl of' tliem'isv at an angle of 45."v with the saidv chord ofthe disk, as clearly shown in Fig; 4 ofthe drawing. The longitudinal axis of thezprisnil 8 is parallel with the chord.

The prisms] and Bfclosely fit within the housing and; against the: diskv 5,' so that when the housing 6. issecured tothe disk 5' by means of the screws? I Il, `the said. prisms will be held securely in they position shownintFig.. 4'.v

Two smal'l-openings-It` and llpassing entirely through the disk 5y areV provided near eachend of the said chord'. of the disk, andl these openings are so positioned that the axis of opening II! produced intersects the interface!! aboutat its middle, and the axisY ot the opening. I -lproducedintersects the total reflecting-surface.I Ill-ofthe prism 8 at about its middle point.

i The centers'ofthe openings I4 and I5 are so locatedion the 4above mentioned chordof the disk 5. that the axes of the holesIII and I5 when projected into thebore 2 of the cannon barrel are parallel withl the axis of` the; bore, and substantially coincident'with the surface of the bore.

Thus, the openingA I5fand. the reector or re- -ecting surface. Illarexso.-disposed that light passing down the bore of the cannon parallel to that part of the surface of the bore adjacent the opening I5, and also parallel to the axis of the bore, is reflected at right angles to its path. whereby it is directed upon: the partially'light",2 reflecting and light transmitting mirrorvorreflector inthe interface 9. This light is then again reected at right angles by the mirror in the interface 9 and proceeds out of the-housing 6 through the opening IL'Iy to theeyeof. the person using the instrument. Also.. it will` be observed that light passing down the bore parallelto that part of the surface adjacent the opening I4, and also parallel to the axis of the bore, will pass through opening I 4 into prismA 1l', and; then through interface 9 and opening I'Iin housing 6 to the eye of the person using the instrument. The mirror in the interface 9 overlapsf'th'e view oi? the periphery of the muzzle end of the boreLZ'"r obtained through the opening I5 upon the view obtained y:through the opening I4. Figs.. 5., 6 and 7 illustratenexamples;ofxsuch overlapped. views.

In Figs. 5 and 6 arcs 2llffmd-'2lV indicatefthe. views of the periphery f of; the,l muzzlezfendiof thei bore obtained'` 'through' openings.: If4`fxand I'5trespectively, and their intersection'. is-Lindicated by.l the numeralZ.

In Fig. 7, the circles `23a-110112llfindicatetheviewsrl of the entire peripheryfof-:gthe muzzle end of thev bore obtained. through openings'y I 4f and.y I Erwhern the eye isrelatiyely close tothe openingvv If'Il in. ther housing Ii. The point; otnintersection'nZS: ofthesea two circles corresponds tor-:therpointsjof intersection 22 in Figs. 5v and' 6.

Openingsv 26, withinA whichthef'thumb'. and, two fingers of: oneihandzmay befinserted, .areprov-ided; in the disk 5 to facilitate .its insertion in 4antirremovalfrom the chamberl 3fofixth'e' cannon. Small?l opening 2:1 is provided? at: ythef'eXact *center oft the' disk 5 when it is desired to lbore sight whenxusing intersecting cords or threadsacross` the muzzle end of the bore, accordingftoxthe known methodf of bore sighting described above.

In use, the bore sight isv inserted inthe cannon as shown in Fig. 1. Then'fthef eyeroftheobserver' or user is placedin line withA the. opening IIS'of the housing 6. The. cannon barrelfis` thenm'ovedi in azimuth and in a vertical'plane until a pointl of intersection 2.2 or 25, is on a'distant target or. on some particular `point thereof, as'. for. example onl the; top of. thev telegraphpolefshown in Fig. 5.. Now the externall sight,4 for `example atelescopic* sight or a collimatorsight, is sighted on the same target or point, and. when this isf accomplished the line of sightof. the. external sightand the axisy of the bore of the cannon. are substantially paralilel. The observer sees two. overlapping images through the bore sight.v Whose. centers arev dis.- placed a distance equal to the distance between the two parallel mirrors 9' and, I0, that is,.thedis.- tance between the centers of holes I4 and I5L` As an example-in a gun' with bore. diameter 'of'.l three inches the displacement'of the intersections of the rim portions' would be about two inches'. Thus, the doubleimage'of" the front of the bore would appear asin Fig. 7', `whenthe'eye` is. positioned very close to the aperturev II. However; a displacement of two-inches'at the` distant target will subtend a negligible-angle at the gun and one which cannot be detected with the `naked*V eye. With the eye at a-greater distance from aperture I'I, two images'(eachdeiined4 by a respectivearc* 2t and 2l) of distant. objects would; overlap so'as to have an area. inacommom thusgiving an 'appear-ance similar-toFigg.A It follows that either of the apparent intersections shown in Figure 7 can be used for bore sighting, with equally accurate results.

In the appended claims, the term gun is used in a broad sense as including cannon, mortars, howitzers, and all caliber ordnance for projecting missiles or projectiles in connection with which the invention can be usefully employed for the purposes described. The term element as used in the claims in connection with the gun bore, means any line lying in the surface of said bore and formed by the intersection therewith of a plane passing through the central longitudinal axis of said bore.

I claim:

1. A bore sighting device comprising a disc adapted to fit within the breech portion of a gun transversely across the longitudinal axis of the bore of said gun, there 4being spaced sighting apertures in said disc, each adapted when said disc is in said breech portion, as aforesaid to define a line of sight parallel to said axis and closely adjacent a respective element of the surface of said bore, an optical prism carried by said disc having parallel end faces extending at 45 across the respective lines of sight, one of said faces being partially reflecting and the other of said faces being totally reflecting, and a complemental prism secured to said partially reflecting face, whereby overlapping views of spaced portions of said bore surface at the muzzle of a gun may be afforded at an observing position at the breech thereof.

2. A bore sight comprising a member adapted to fit within and against the Walls of the breech chamber of a gun in a plane normal to the axis of the bore of said gun, a reflector carried by said member so as to lie upon a chord of the cross section of said chamber adjacent said member, in position to reflect at right angles, along said chord, an image of a rst portion of the muzzle end of the bore of said gun, a partial reflector element carried by said member in position to deflect said image rearwardly parallel to the bore axis while simultaneously transmitting parallel with said bore a direct view of a second portion of the muzzle end of the bore of said gun, whereby overlapping views of said portions may be simultaneously observed at a position rearwardly of said member.

3. In a bore sighting device for guns, disc means adapted to be positioned within the breech portion of the bore of a gun in predetermined transverse relation to the axis of said bore, said disc having means which, when said disc is so positioned, defines spaced lines of sight each parallel to said axis and closely adjacent a respective element of the surface of said bore, and optical prism means carried by said disc means and adapted to project along one said line of sight overlapping images of muzzle portions of said bore adjacent the respective elements and to project said overlapping images to an observing position at the breech of said gun.

4. A bore sight comprising a disc adapted to fit within the breech chamberI of a oun transversely across the bore axis of said gun, said disk having first and second openings adjacent the periphery thereof whose centers determine a chord of a cross section of the bore of said gun, a rst reflector carried by said disk over the iirst said opening r and positioned to reflect at right angles parallel to said chord an image of a first portion of the muzzle of thg gun, a partial reector carried by said disk over said second opening and positioned to reflect said image rearwardly along a line parallel to the axis of the bore of said gun and an optical element cooperating with said partial reflector to transmit rearwardly an image of a second portion of the muzzle of said gun adjacent said line, whereby overlapping images of said portions may be simultaneously observed.

5. In an instrument for bore sighting a gun, a frame adapted to be positioned across the breech portion of the bore of said gun in fixed relation with said gun, means carried by said frame and deining, when said frame is positioned as aforesaid, first and second spaced parallel sight lines each closely adjacent the surface of said bore and parallel with the axis thereof, first means comprising an optical prism carried by said frame to project to a viewing position at the breech of the gun, a first view of the muzzle portion of the gun bore adjacent said first line, and second means carried by said frame and comprising an optical prism cooperating with said first prism to project into overlapping relation with said rst View at said position, a second View of the muzzle portion of said gun proceeding along said second line, said overlapping views including an image of intersecting rim sections of said muzzle portions to thereby provide a Sighting point.

LEO I-I. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,284,831 McCanlies June 2, 1942 2,422,181 Brown Feb. 17, 1947 

